Did you update SFDC or Marketo with those 600K leads?
Typically Marketo sync checks every 5 minutes. My understanding is that it can take 1-5 days for larger databases to sync, especially for a first sync...which is why I always do this over a weekend.
You can check the Notification logs or any Lead Activiy report. I don't recall seeing a "system log", but you can ask Support to check on this for you.